This revised application requests funds to continue studies of separate site catalysis by the enzyme pyruvate phosphate dikinase (PPDK). PPDK catalyzes the interconversion of ATP, Pi, and pyruvate with AMP, PPi and PEP by using two separate active sites linked by a carrier histidine residue. The overall goal of the proposed studies is to determine the mechanism by which the carrier histidine is transferred between active sites through precisely oriented and timed domain-domain docking steps. These studies will provide a deeper understanding of the forces controlling transient protein-protein complex formation in signal transduction pathways and template directed biosynthetic pathways. Five specific aims are listed. These are (1) determine the X-ray crystal structure of PPDK conformer 2, (2) determine the role of domain linkers in facilitating successful domain-domain docking, (3) determine the role of interactions between domain-domain interface residues in facilitating correct docking orientation and in optimizing residence time, (4) distinguish between a through-solvent-domain-diffusion model and a sliding-domain model and (5) identify the role of the PPDK homologue in Mycobacterium tuberculosis. This last aim will be carried out for the purpose of discovering a novel metabolic pathway operating with in this pathogen, as well as a novel target for drug design.